1. Field of the Invention
This invention relates to a failure determination device for a humidity sensor that detects humidity of exhaust gases from an internal combustion engine and a control system for controlling an exhaust passage changeover valve capable of switching an exhaust passage for exhaust gases from the engine between a main exhaust passage and a bypass exhaust passage having an adsorber capable of adsorbing hydrocarbons and water contained in the exhaust gases.
2. Description of the Prior Art
Generally, in the exhaust system of a gasoline engine or the like, a three-way catalyst is arranged in an intermediate portion of the exhaust pipe in order to purify harmful substances (hydrocarbons, carbon monoxide and nitrogen compounds) in exhaust gases. However, immediately after the engine is started in a cold condition (e.g. before approximately 30 to 40 seconds have passed after the cold start of the engine), for instance, the three-way catalyst remains inactive, and the harmful substances cannot be purified sufficiently, so that particularly hydrocarbons are emitted from the engine as unburned combustible components. Therefore, in order to prevent emission of such hydrocarbons into the air, there has been proposed an engine which has, in addition to the three-way catalyst, an adsorber arranged in the exhaust pipe, for adsorbing hydrocarbons. Such an adsorber is arranged in a bypass exhaust passage which is branched from an intermediate portion of a main exhaust passage of the exhaust pipe having the three-way catalyst arranged therein. A changeover valve arranged in a bifurcating portion of the exhaust pipe switches the exhaust passage for exhaust gases between the main exhaust passage and the bypass exhaust passage. Thus, exhaust gases emitted immediately after the cold start of the engine are purified by the adsorber adsorbing hydrocarbons, and then discharged into the air.
The above-mentioned adsorber carries a zeolite on its surface. When exhaust gases are passing through the bypass passage, the molecules of hydrocarbons enter small holes of the zeolite whereby the hydrocarbons are adsorbed by the adsorber. The adsorber of this kind desorbs hydrocarbons once adsorbed thereby when it is heated to a temperature equal to or higher than a predetermined temperature (e.g. 100 to 250xc2x0 C.). The desorbed hydrocarbons are recirculated to the engine e.g. via the EGR. As described above, the adsorber repeatedly carries out adsorption and desorption of hydrocarbons. However, the amount of undesorbed hydrocarbons permanently remaining in the adsorber may progressively increase or the small holes of the adsorber may be destroyed by a long-term use thereof. This results in the degradation of the adsorber, that is, a lowered adsorbing capacity of the adsorber for adsorbing hydrocarbons. If the engine is repeatedly started in such a state, an increasing amount of unadsorbed hydrocarbons is emitted into the air. Therefore, to carry out engine control for desorbing hydrocarbons (e.g. by elevating the temperature of the adsorber) to cope with the degraded state of the adsorber, or to notify the driver of the degradation of the adsorber, the present assignee has already proposed a degradation-detecting device for detecting degradation of a hydrocarbon adsorber by Japanese Patent Application No. 2000-66443.
In this degradation-detecting device, a humidity sensor is arranged at a location downstream of an adsorber in a bypass exhaust passage such that the humidity of exhaust gases having passed through the adsorber is detected by the humidity sensor, and degradation of the adsorber is detected based on the result of the detection. This degradation-detecting device utilizes proportionality between the adsorber""s capabilities of adsorbing hydrocarbons and of adsorbing water. By detecting the humidity of exhaust gases having passed through the adsorber by using the humidity sensor, it becomes possible to detect lowered adsorbing capability of the adsorber for adsorbing hydrocarbons and water, that is, degradation of the adsorber.
Although the above degradation-detecting device can properly detect the degradation, once the humidity sensor of the degradation-detecting device has failed, the proper detection of the degradation cannot be performed, making it impossible to suitably carry out engine control or notify the driver of the degradation of the adsorber. Therefore, the conventional degradation-detecting device has room for improvement in this respect.
A control system of the above-mentioned kind has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 11-2115. The proposed control system includes a temperature sensor disposed in the exhaust pipe at a location between the three-way catalyst and the changeover valve, for detecting an exhaust temperature on a downstream side of the three-way catalyst, and based on the result of detection by the temperature sensor, controls the changeover valve in the following member: An exhaust temperature detected by the temperature sensor, and a predetermined temperature (e.g. 300xc2x0 C.) defined in advance are compared with each other. When the exhaust temperature is lower than the predetermined temperature, it is determined that the three-way catalyst remains inactive, and the main exhaust passage is closed and at the same time the bypass exhaust passage is opened by the changeover valve. Thus, the exhaust gases are guided into the bypass exhaust passage, for causing hydrocarbons in the exhaust gases to be adsorbed by the adsorber, whereby the exhaust gases are purified. On the other hand, when the exhaust temperature is higher than the predetermined temperature, it is determined that the three-way catalyst has been activated, and the bypass exhaust passage is closed and at the same time the main exhaust passage is opened by the changeover valve, whereby the exhaust gases purified by the three-way catalyst are emitted without farther processing. It should be noted that the above adsorber adsorbs hydrocarbons when it is in a low temperature condition (e.g. lower than 100xc2x0 C.) while desorbing the hydrocarbons once adsorbed thereby at temperatures equal to or higher than a predetermined temperature (e.g. 100 to 250xc2x0 C.). The desorbed hydrocarbons are recirculated to the engine e.g. via an EGR pipe.
As described hereinabove, in the control system for the changeover valve, the exhaust temperature on the downstream side of the three-way catalyst is detected by the temperature sensor, and the condition of the three-way catalyst is estimated based on the result of the detection so as to control the changeover valve. In the above control system, however, the changeover valve is controlled irrespective of the actual state of adsorption of hydrocarbons by the adsorber. Moreover, generally, a temperature detected by the temperature sensor is slow in change, has low responsiveness, and is liable to be adversely affected by parameters including an outside air temperature, and the like. Therefore, in the above control system, it is sometimes impossible to control the changeover valve with appropriate timing, which causes insufficient purification of exhaust gases.
It is a first object of the invention to provide a failure determination device which is simple in construction and capable of properly determining failure of a humidity sensor that detects the humidity of exhaust gases from an internal combustion engine.
It is a second object of the invention to provide a control system for an exhaust passage changeover valve, which is capable of switching the changeover valve with appropriate timing, thereby making it possible to sufficiently purify exhaust gases.
To attain the first object, according to a first aspect of the invention, there is provided a failure determination device for a humidity sensor that detects humidity of exhaust gases from an internal combustion engine.
The failure determination device according to the first aspect of the invention is characterized by comprising:
operating condition-detecting means for detecting an operating condition of the engine;
failure determination execution-judging means for judging whether or not the engine is in an operating condition in which failure determination of the humidity sensor can be executed, based on a result of detection by the operating condition-detecting means; and
humidity sensor failure-determining means for determining whether or not the humidity sensor has failed, based on a result of detection by the humidity sensor, when the failure determination execution-judging means judged that the failure determination of the humidity sensor can be executed.
According to this failure determination device, when the failure determination execution-judging means judged that the engine is in an operating condition in which failure determination of the humidity sensor can be executed, depending on the operating condition of the engine, the humidity sensor failure-determining means determines whether or not the humidity sensor has failed, based on the result of detection by the humidity sensor. Depending on the operating condition of the engine, if a value of humidity detected by the humidity sensor is not within a predetermined range in spite of the engine being in a predetermined operating condition wherein the detected value should be within the predetermined range, it is possible to determine that the humidity sensor has failed. This makes it possible to properly determine failure of the humidity sensor with appropriate timing. Further, the determination of failure of the humidity sensor is carried out by using a humidity value detected by the humidity sensor, and hence it is possible to implement a failure determination device for determining a failure of the humidity sensor by relatively simple construction without any need for a special device.
Preferably, an exhaust passage for the exhaust gases is configured such that the exhaust passage can be switched between a main exhaust passage and a bypass exhaust passage having an adsorber arranged in an intermediate portion thereof, the adsorber being capable of adsorbing hydrocarbons and water in the exhaust gases, the humidity sensor being arranged at a location downstream of the adsorber in the bypass exhaust passage, and the failure determination execution-judging means judges that the failure determination of the humidity sensor can be executed when the exhaust passage has been switched to the bypass exhaust passage, and at the same time the adsorber is adsorbing hydrocarbons in the exhaust gases guided into the bypass exhaust passage.
According to this preferred embodiment, the humidity sensor is arranged at a location downstream of the adsorber in the bypass exhaust passage, and hence it is possible to detect the humidity of exhaust gases having passed through the adsorber which is performing adsorbing operation, more specifically, the humidity of exhaust gases (post-adsorption exhaust gases) whose hydrocarbons and water have been adsorbed by the adsorber. Thus, when the exhaust gases are guided into the bypass exhaust passage and at the same time the adsorber is adsorbing hydrocarbons in the exhaust gases, the failure determination execution-judging means judges that failure determination of the humidity sensor can be executed. Since the adsorber""s adsorbing capabilities of adsorbing hydrocarbons and of water are proportional to each other, the humidity of the post-adsorption exhaust gases has close correlation with an actual state of adsorption of hydrocarbons by the adsorber. Therefore, so long as the adsorber is adsorbing hydrocarbons, a value of the humidity detected by the humidity sensor should be within the predetermined range e.g. when the adsorption of hydrocarbons is nearing completion. This enables the failure of the humidity sensor to be determined based on a value of the humidity detected by the humidity sensor. As described hereinabove, when the adsorber is adsorbing hydrocarbons, it is determined that the engine is an operating condition suitable for carrying out failure determination of the humidity sensor, whereby failure of the humidity sensor can be determined properly.
More preferably, the hydrocarbons are desorbed from the adsorber by switching the exhaust passage to the main exhaust passage, and the failure determination device further comprises desorption state-detecting means for detecting a state of desorption of the hydrocarbons from the adsorber, the failure determination execution-judging means judging whether or not the failure determination of the humidity sensor can be executed, based on the state of the desorption of the hydrocarbons from the adsorber at a time of termination of a preceding operation of the engine, detected by the desorption state-detecting means.
According to this preferred embodiment, it is judged whether or not failure determination of the humidity sensor can be executed, based on a state of desorption of the hydrocarbons from the adsorber, which is detected by the desorption state-detecting means at a time of termination of a preceding of operation of the engine. Therefore, appropriate failure determination can be executed by taking into account the state of desorption of hydrocarbons from the adsorber, which affects a value of humidity detected by the humidity sensor. More specifically, for instance, if desorption of hydrocarbons had not been completed during the immediately preceding operation of the engine, manners of changes in values detected by the humidity sensor become different so as to advance or retard timing of proper execution of the failure determination from a predetermined one. In such a case, according to this preferred embodiment, the failure determination is not executed, but only when desorption of hydrocarbons had been completed during the immediately preceding operation of the engine, the failure determination is caused out, whereby it is possible to prevent erroneous failure determination of the humidity sensor.
More preferably, the failure determination execution-judging means further includes timer means for measuring a time period after a start of the engine, and determines whether or not failure determination of the humidity sensor can be executed further based on the time period measured by the timer means.
Further preferably, the failure determination execution-judging means further includes fuel injection time-integrating means for calculating a cumulative value of fuel injection time periods after a start of the engine, and determines whether or not failure determination of the humidity sensor can be executed further based on the cumulative value calculated by the fuel injection time-integrating means.
Preferably, the humidity sensor failure-determining means determines that the humidity sensor has failed when the value of humidity detected by the humidity sensor is lower than a predetermined value.
To attain the second object, according to a second aspect of the invention, there is provided a control system for controlling an exhaust passage changeover valve of an internal combustion engine, the changeover valve switching an exhaust passage having a catalytic device arranged therein for purifying exhaust gases from the engine between a main exhaust passage and a bypass exhaust passage bypassing the main exhaust passage and having an adsorber arranged therein which is capable of adsorbing hydrocarbons and water in the exhaust gases.
The control system according to the second aspect of the invention is characterized by comprising:
a humidity sensor arranged at a location downstream of the adsorber in the bypass exhaust passage, for detecting humidity of the exhaust gases guided into the bypass exhaust passage; and
changeover valve drive means for driving the changeover valve based on a result of detection by the humidity sensor.
According to this control system, the humidity sensor is arranged at a location downstream of the adsorber in the bypass exhaust passage, and hence it is possible to detect the humidity of exhaust gases having passed through the adsorber which is performing adsorbing operation, more specifically, the humidity of exhaust gases (post-adsorption exhaust gases) whose hydrocarbons and water have been adsorbed by the adsorber. Depending on the result of detection by the humidity sensor, the changeover valve is driven so as to switch the exhaust passage between the main exhaust passage and the bypass exhaust passage. Since the adsorber""s capabilities of adsorbing hydrocarbons and of water are proportional to each other, the humidity of the post-adsorption exhaust gases has close correlation with an actual state of adsorption of hydrocarbons by the adsorber. Therefore, by detecting the humidity of the post-adsorption exhaust gases, it is possible to properly estimate whether or not the adsorption of hydrocarbons by the adsorber has been actually completed i.e. carried out to the limit of its capacity, and by controlling the switching operation of the changeover valve depending on the estimation, it is possible to switch the exhaust passage with appropriate timing, whereby the exhaust gases can be sufficiently purified.
Preferably, the control system further comprises delayed response compensation means for compensating for a delayed response of the humidity sensor.
According to this preferred embodiment, the delayed response of the humidity sensor is compensated for by the delayed response compensation means, whereby even if a humidity sensor actually used has low responsiveness, it is possible to control the switching operation of the changeover valve with more appropriate timing by compensating for the delayed response of such a humidity sensor.
Preferably, the humidity sensor includes a sensor element for being exposed to the exhaust gases for detecting humidity thereof, and the control system further comprises a heater for heating the sensor element, operating condition-detecting means for detecting an operating condition of the engine, and heater control means for controlling an operation of the heater depending on the operating condition detected by the operating condition-detecting means.
According to this preferred embodiment, the heater control means drives the heater depending on the operating condition of the engine, and the heater heats the sensor element of the humidity sensor, whereby the condition of the sensor element can be made suitable for detecting the humidity of the exhaust gases. For instance, there is a fear that condensation forms on the sensor element and coke or soot is deposited on the same to hinder proper detection of the humidity of the sensor element. Therefore, when the engine is in a condition in which the condensation and deposition are liable to occur, by heating the sensor element, the above inconveniences can be avoided, which enables the sensor to properly detect the humidity.
Preferably, the control system further comprises upstream-side humidity-estimating means for estimating humidity at a location upstream of the adsorber, based on a value of the humidity detected by the humidity sensor.
More preferably, the changeover valve drive means drives the changeover valve based on a difference between the value of the humidity detected by the humidity sensor and a value of the humidity estimated by the upstream-side humidity-estimating means.
Further preferably, the changeover valve drive means drives the changeover valve further based on a cumulative value of the difference between the value of the humidity detected by the humidity sensor and the value of the humidity estimated by the upstream-side humidity-estimating means.
Preferably, the control system further comprises an upstream-side humidity sensor for detecting humidity at a location upstream of the adsorber.
More preferably, the changeover valve drive means drives the changeover valve based on a difference between a value of the humidity detected by the humidity sensor and a value of the humidity detected by the upstream-side humidity sensor.
Further preferably, the changeover valve drive means drives the changeover valve further based on a cumulative value of the difference between the value of the humidity detected by the humidity sensor and the value of the humidity detected by the upstream-side humidity sensor.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.